This application is the US national phase of international application PCT/GB00/04323 filed Nov. 13, 2002, which designated the US.
The invention relates to surfaces which are textured (e.g. which exhibit patterns of grooves) and which can be used as absorbers, selective reflectors and radiation detectors. The invention has utility in a wide range of application such as coatings on aircraft to reduce radar signature (i.e. stealth application) solar absorbers, blackened metals for visual camouflage, improved radiation detectors, anti-counterfeiting markings, electromagnetic filters etc. This list is not exhaustive.
More particularly the invention utilises the coupling of photons to modes known as surface plasmon polaritons (SPPs). Surface plasmon polaritons are surface charge density oscillations that are created at the interface between a metallic and a dielectric material. It is a composite of an electromagnetic wave in the dielectric and a plasma wavexe2x80x94a collective electronic excitationxe2x80x94in the metal. Light polarised such that its electric vector crosses such an interface can couple into a SPP and be absorbed but this is not possible under normal conditions: photons made directly incident onto the surface can never possess sufficient momentum to excite the SPP, and a momentum increase from a prism or diffraction grating is required. Under such conditions there are certain frequencies and angles of incidence (corresponding to momentum values) for which SPP""s can be observed, these being dictated by the electromagnetic properties of the materials in the vicinity of the interface. Perturbations of these conditions (e.g. deposition of a different dielectric film at the interface) will alter these values, and this phenomenon has formed the basis of several chemical and biological sensors.
U.S. Pat. No. 5,598,267 discloses an optical sensing device that incorporates a surface plasmon polariton apparatus for converting radiation between s and p modes together with a sensor for detecting a maximum in conversion between the s and p modes.
As stated above, on a flat metal surface, an SPP mode cannot be directly coupled to by incident radiation because the photons do not exhibit high enough momentum values. This can be illustrated on a plot of in-plane momentum versus photon frequency (i.e. the dispersion curve of the SPPxe2x80x94FIG. 1): the SPP mode momentum is represented by a curve that lies outside the light cone (the momentum values obtainable from the incident photons). The momentum of a photon k(xcfx89) is linearly proportional to its frequency xcfx89xe2x80x94hence the light conexe2x80x94but the x-axis represents the component of momentum parallel to the surface. (The light cone represents the momentum values attainable by photons in vacuo). Hence for a given frequency xcfx891, the photon momentum along the surface can vary from zero (normal incidence) to k(xcfx891) (grazing incidence), and the x-axis is therefore proportional to the incident angle.
When a profiled grating is created on the metal surface, two closely related physical effects occur. First the periodicity of the grating provides additional in-plane momentum for the incident photons, effectively shifting the SPP curve into the light cone, which enables the excitation of the SPP by electromagnetic radiation. Secondly, the SPP dispersion curve is often split into bands, just as the electronic states in a periodic potential form into bands. These two effects enable workers to design the optical properties of metal surfaces via choosing suitable grating profiles.
Gratings that exhibit pitches (i.e. groove widths in a periodic structure) that are far less than the wavelength of the incident radiation cannot diffract the radiation, and the radiation is specularly reflected (i.e. it is reflected in a mirror-like fashion). Such gratings are sometimes defined to exhibit zero-order diffraction, since no additional beams or orders are created. It has recently been stated in the literature that very flat (i.e. parallel to the x-axis in the dispersion plot) SPP bands may be formed on deep short pitch metal gratings of this type, each of which corresponds to a standing SPP mode localized in the grating grooves. However, within the light cone the SPP modes on such a sinusoidal grating are very broad (short ranged) because they are strongly radiative. Consequentially the excitation of the SPP modes only leads to weak absorption of the incident light over a wide range of the frequency band. In other words a single-period zero-order sinusoidal grating does not drastically change the optical properties of the metal surface except for the case of large incident angle where sharp resonant absorption may occur.
However the inventors have determined that if two gratings are superimposed on a surfacexe2x80x94one of short pitch (a zero-order (non-diffractive) grating) and one being of longer period (possibly diffractive)xe2x80x94then the band structure can be controlled in a unique fashion. The inventors have thus determined that by using a grating having two or more periodic profiles it is possible to enhance the SPP effects to limit reflection further, producing absorption over wide ranges of angle of incidence and/or frequency. See xe2x80x9cOptical characterization of a complex grating profilexe2x80x9d, R. A. Watts et al., Journal of modern optics, vol 45, no 3, March 1998, pages 639-651 where an example of a complex grating is studied.
The invention comprises a surface capable of supporting surface charge oscillations and exhibiting a profiled grating, said profile comprising at least two superposed periodic profiles. Preferably, the at least two superimposed periodic profiles have different periodicity. In order to produce a surface plasmon it is necessary that the surface (i.e. substrate) has free electrons and thus the surface is necessarily metallic or a semiconductor. It may be coated in a layer of dielectric material.
The use of two superimposed periodic profiles creates a textured surface.
Preferably, the profiled grating comprises two superposed periodic profiles, a first periodic profile having a shorter periodic profile and a second period having a longer periodic profile. Preferably, the amplitude of said shorter periodic profile is greater than the amplitude of the larger periodic profile. Advantageously the period of the smaller grating period is non-diffractive, and less than twice the wavelength of incident radiation, preferably less than a third of the wavelength of incident radiation.
The first grating""s features should be too finely spaced to create diffracted orders, exhibiting a pitch that is less than the wavelength of the incident radiation i.e. it should be a zero-order grating. The splitting of the SPP bands will be dictated by the depth of the grooves of this short-period grating, and the preferred sample geometry will exhibit deeper short-pitch grooves than the second long-pitch grating. When the bands are split they become xe2x80x98flatxe2x80x99 (i.e. tend towards being parallel to the momentum axis of the dispersion graph) and hence can be accessed over a wide range of angles of incidence for a given frequency.
The second grating is of a longer pitch, and may or may not be fully diffractive. The purpose of this second grating is to provide efficient overlap of the split SPP bands with the light-cone, and if this can be achieved without recourse to a fully diffractive structure then no radiation will be lost (i.e. transmitted away from the surface) in diffracted beams.
By choosing the correct pitches, groove depths, profile shapes and material constants it is possible to modify the usual xe2x80x9cnarrow anglexe2x80x9d absorption of an SPP excited on a single-period diffractive surface into a wide angle feature that occurs at several different frequencies. Furthermore if the different frequencies are sufficiently close to each other in value then the absorption features will overlap creating a wide frequency absorption too.
Thus such surfaces can provide for very wide spectral absorbers over a wide range of incident angle ranges, as well as efficient narrow band absorbers of tailored properties.
The effect of textured surfaces is also dependent on the angle of polarisation of the incident light. Only radiation which is electrically polarised in a plane perpendicular (orthogonal) to the surface will have the desired effect and thus the invention provides for allowing radiation polarised in a particular direction to be reflected whilst damping radiation polarised in a different direction.
In a more complex embodiment of the invention the grating essential provides for the double frequency grating in two dimensions. For example the double frequency profile is fabricated in two azimuthal directions across the surface e.g. one perpendicular to the other. This will reduce the polarisation dependence of absorption.